Device for measuring a gas flow, and method for using the device

ABSTRACT

A device for measuring a gas flow (F) has a measuring body (1) in the form of a cylinder, and a heating device (12) for heating the measuring body. A slot (5) extends axially into the cylinder from one end thereof so as to divide a first portion of the cylinder into two symmetrical halves (6a, 6b). The heating device (12) is disposed in a second cylinder portion which is not divided by the slot (5). A temperature sensor (13) is arranged in the second cylinder portion, and a temperature sensor (14, 15) is arranged in each cylinder half (6a, 6b). For measuring a gas flow (F), the measuring body (1) is so disposed in the gas flow that the slot (5) extends at right angles to the gas flow with one half (6a) facing the gas flow and with the other half (6b) facing away from the gas flow. The measuring body (1) is heated by the heating device (12) to a temperature exceeding the ambient temperature by a predetermined value. The temperature is measured in each of the two symmetrical cylinder halves (6a, 6b), whereupon the gas flow is calculated on the basis of the measured temperatures.

FIELD OF THE INVENTION

The present invention relates to a device designed for measuring a gasflow and having a measuring body to be placed in the gas flow, and aheating means for heating the measuring body in relation to theenvironment. The invention also relates to a method for using thedevice.

DESCRIPTION OF THE BACKGROUND ART

A typical spray booth line for painting car bodies is divided into aplurality of successive sections which by means of fans are suppliedwith vertical supply air flows through filter roofs provided in thesections. In those sections where the car bodies are sprayed with paint,exhaust air flows polluted with paint particles and solvents areevacuated by means of fans. In order that polluted air should not spreadhorizontally from the spray sections to the other sections, it is vitalthat the horizontal air flows between the sections be directed towardsthe spray sections. It is also vital that the horizontal air flows besmall in relation to the supply and exhaust air flows so as not tocreate secondary eddies in the spray sections or entail that paintparticles are transferred from one car body to another. The horizontalair flows are suitably regulated by such a speed control of the supplyair fans that the size and the direction of the horizontal air flowswill become as desired. To permit carrying out this control in optimumfashion, the horizontal air flows must be accurately measured.

Thermal flow transducers, i.e. transducers having a measuring body whichshould be placed in the flow to be measured, and a means for heating themeasuring body, and relying in different ways on the fact that the flowcools the heated measuring body, are known in various designs. However,none of the prior-art flow transducers can be considered to comply withthe requirements 1-3 which are listed below and which must be placed ona flow transducer to be used in a spray booth line of the type describedabove.

1) Since a spray booth line is classified as explosive environment, themeasuring body of the flow transducer may only be heated to atemperature which but insignificantly exceeds the ambient temperature.

2) The flow transducer must be relatively insensitive to deposits ofpaint and other chemicals on the measuring body.

3) The flow transducer must have a high sensitivity in the range aroundzero, especially in the range -2 m/s-+2 m/s.

SUMMARY OF THE INVENTION

The main object of the present invention is to provide a flow transducermeeting these requirements.

This object is achieved by means of a device which is of the typedescribed by way of introduction and which according to the presentinvention is characterised in that the measuring body is in the form ofa cylinder which is symmetrical with respect to a plane of symmetryextending through the axis of the cylinder, that a slot, containing saidplane of symmetry, extends axially a certain distance into the cylinderfrom one end thereof, so as to divide a first portion of the cylinderinto two halves which are symmetrical with respect to said slot, thatthe heating means is provided at a distance from the bottom of the slotin a second portion of the cylinder which is not divided by the slot,that a temperature sensor is provided in said second portion of thecylinder adjacent the bottom of the slot for measuring the temperatureto which the measuring body is heated by the heating means, and that atemperature sensor is provided in each of the symmetrical cylinderhalves adjacent said one end of the cylinder for measuring thetemperature in the respective half.

The measuring body preferably is in the form of a straight circularcylinder.

Another object of the present invention is to provide a suitable methodfor using this device.

According to the invention, this object is achieved by a method which ischaracterised by so disposing the measuring body in the gas flow thatthe slot extends at right angles to the gas flow with one half facingthe gas flow and with the other half facing away from the gas flow,heating the measuring body by the heating means to a temperatureexceeding the ambient temperature by a predetermined value, measuringthe temperature in each of the two symmetrical cylinder halves, andcalculating the gas flow on the basis of the measured temperatures.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to theaccompanying drawings which are given by way of illustration only, andthus are not limitative of the present invention.

FIG. 1 is an axial longitudinal section of a measuring device accordingto the present invention.

FIG. 2 is an axial longitudinal section of the measuring device at rightangles to the section in FIG. 1.

FIG. 3 is an end view showing the measuring device, with the heatingmeans removed, from the left in FIG. 1.

FIG. 4 is an end view showing the measuring device from the right inFIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The measuring device shown in the drawing has a measuring body 1 formedas a single piece and consisting of brass. The measuring body 1 is inthe form of a straight, circular cylinder which at one end has acircumferential flange 2 and a grooved portion 3 located inwardly of theflange. The flange 2 and the grooved portion 3 are used for fixing thedevice in a wall 4, through which the device extends into a gas flow F,e.g. an air flow, that is to be measured.

At its end opposite the flange 2, the measuring body 1 has a diametricalslot 5 extending axially from this end a certain distance into themeasuring body 1. Thus, the slot 5 divides a portion of the cylinderinto two similar halves 6a and 6b. As seen in FIG. 1, the measuring body1 is so disposed in the gas flow F that the slot 5 will extend at rightangles to the gas flow, with one cylinder half 6a facing the gas flowand with the other cylinder half 6b facing away from the gas flow.

Three axially directed shoulder bores 7, 8 and 9, whose center lines arelocated at the same distance from the axis of the cylinder and offset90° with respect to each other (see FIG. 3), extend into the measuringbody 1 from its flanged end. The shoulder bores 7 and 8 are locateddiametrically opposite each other, and the connecting line between theircenter lines is perpendicular to the slot 5 (see FIG. 3). The shoulderbores 7 and 8 each extend equally far into one cylinder half 6a and 6b,respectively, to a position just inside the slotted end of the measuringbody 1. The shoulder bore 9 is located opposite the slot 5 (see FIG. 3).However, it does not extend as far as the slot, but ends a shortdistance from the bottom 10 of the slot (see FIG. 2).

A shoulder bore 11 extends along the axis of the cylinder some distanceinto the measuring body 1 from the flanged end thereof. The inner,narrower portion of the bore 11 is threaded (not shown). A heating means12 is screwed into the shoulder bore 11 and, additionally, is fixedtherein by means of an epoxy glue having good thermal conductivity. Theheating means 12, containing a heating resistor (not shown), is adaptedto heat the measuring body 1 to a temperature exceeding the ambienttemperature by a predetermined, relatively low value, e.g. 10° C.

The temperature of the measuring body 1 is measured by means of atemperature sensor 13 disposed at the bottom of the shoulder bore 9. Inthis case, the temperature sensor 13 is a resistance thermometer with aplatinum resistance wire having a resistance of 100 Ω at 0° C.

The temperature of the two cylinder halves 6a and 6b is measured bymeans of two temperature sensors 14 and 15 disposed at the bottom of theshoulder bores 7 and 8, respectively. Each of the temperature sensors 14and 15 here consists of two resistance thermometers, each having aplatinum resistance wire of a resistance of 100 Ω at 0° C.

The shoulder bores 7, 8 and 9 are filled with epoxy glue having goodthermal conductivity. The four resistance thermometers in thetemperature sensors 14 and 15 are connected in a measuring bridge viaconnecting wires (not shown) running from the sensors to the outside ofthe measuring body 1 via the epoxy Glue in the shoulder bores 7 and 8.The connecting wires (not shown) of the temperature sensor 13 similarlyrun from the sensor to the outside of the measuring body 1 via the epoxyGlue in the bore 9.

When the measuring device is to be used, the measuring body 1 is placedin the gas flow F in the manner shown in FIG. 1. The measuring body 1 isheated to a temperature which is 10° C. above the ambient temperature,which is measured by a separate temperature transducer. The gas flow Fcools the measuring body 1, the cylinder half 6a facing the gas flow Fbeing cooled to a greater extent than the other cylinder half 6b. Thetemperature difference between the two cylinder halves 6a and 6b ismeasured by means of the temperature sensors 14 and 15, whereupon theGas flow is calculated on the basis of the measured temperaturedifference.

Theoretically, this temperature difference dT is a function of severalparameters:

    dT=f(αA1/KA2,Q,T)

where α is the coefficient of heat transfer between the measuring body 1and the gas, K is the heat conductivity of the measuring body 1, A1 isthe area of the circumferential surface of the portion of the measuringbody 1 through which the slot 5 extends, A2 is the cross-sectional areaof the measuring body 1, Q is the gas flow, and T is the differencebetween the temperature of the measuring body 1 and the ambienttemperature. dT is proportional to αA1/KA2. Since A1=π·d·L, where d isthe diameter of the measuring body 1 and L is the depth or axial lengthof the slot 5, and A2=πd² /4, it is possible in actual practice toincrease the sensitivity of the measuring device by increasing, withincertain limits, the depth L of the slot 5 and/or decreasing the diameterd of the measuring body 1.

The invention is not restricted to the embodiment described above, butmay be modified in several different ways within the scope of theaccompanying claims. For instance, the measuring body is not necessarilya straight, circular cylinder, but may be a cylinder otherwise designed.However, the cylinder should be symmetrical with respect to a plane ofsymmetry extending through the axis of the cylinder, such that the slotcan divide a portion of the cylinder into two halves which aresymmetrical with respect to the slot.

We claim:
 1. A device designed for measuring a gas flow comprising ameasuring body to be placed in the gas flow, heating means for heatingthe measuring body in relation to the environment, the measuring body inthe form of a cylinder which is symmetrical with respect to a plane ofsymmetry extending through the axis of the cylinder, a slot containingsaid plane of symmetry and extending axially a certain distance into thecylinder from one end thereof, so as to divide a first portion of thecylinder into two halves which are symmetrical with respect to said slotthe heating means at a distance from the bottom of the slot in a secondportion of the cylinder which is not divided by the slot a temperaturesensor in said second portion of the cylinder adjacent the bottom of theslot for measuring the temperature to which the measuring body is heatedby the heating means, and that a temperature sensor in each of thesymmetrical cylinder halves adjacent said one end of the cylinder formeasuring the temperature in the respective half.
 2. Device as claimedin claim 1, wherein the measuring body is in the form of a straightcircular cylinder.
 3. A method for using the device as claimed in claim1 for measuring a gas flow, comprising: disposing the measuring body inthe gas flow so that the slot extends at right angles to the gas flowwith one half facing the gas flow and with the other half facing awayfrom the gas flow, heating the measuring body by the heating means to atemperature exceeding the ambient temperature by a predetermined value,measuring the temperature in each of the two symmetrical cylinderhalves, and calculating the gas flow on the basis of the measuredtemperatures.